Classical Be Stars: Rapidly Rotating B Stars with Viscous Keplerian Decretion Disks (1310.3962v1)

Abstract: In the past decade, a consensus has emerged regarding the nature of classical Be stars: They are very rapidly rotating main sequence B stars, which, through a still unknown, but increasingly constrained process, form an outwardly diffusing gaseous, dust-free Keplerian disk. In this work, first the definition of Be stars is contrasted to similar classes, and common observables obtained for Be stars are introduced and the respective formation mechanisms explained. We then review the current state of knowledge concerning the central stars as non-radially pulsating objects and non-magnetic stars, as far as it concerns large scale, i.e., mostly dipolar, global fields. Localized, weak magnetic fields remain possible, but are as of yet unproven. The Be phenomenon, linked with one or more mass ejection processes, acts on top of a rotation rate of about 75% of critical or above. The properties of the process can be well constrained, leaving only few options, most importantly, but not exclusively, non-radial pulsation and small scale magnetic fields. Of these, it is well possible that all are realized: In different stars, different processes may be acting. Once the material has been lifted into Keplerian orbit, memory of the details of the ejection process is lost, and the material is governed by viscosity. The disks are fairly well understood in the theoretical framework of the viscous decretion disk model. This is not only true for the disk structure, but as well for its variability, both cyclic and secular. Be binaries are reviewed under the aspect of the various types of interactions a companion can have with the circumstellar disk. Finally, extragalactic Be stars, at lower metallicities, seem more common and more rapidly rotating.

• The paper consolidates the current understanding of Classical Be stars, focusing on their rapid rotation, viscous Keplerian decretion disks, underlying mechanisms, and astrophysical implications.
• Classical Be stars are the most rapid non-degenerate stellar rotators, with circumstellar disks primarily governed by viscous decretion and characterized by specific radial density fall-offs.
• New observational techniques like interferometry provide geometric constraints on disks, while metallicity and binary interactions play key roles in their evolution and observed properties.

An Expert Overview of "Classical Be Stars: Rapidly Rotating B Stars with Viscous Keplerian Decretion Disks"

The paper on "Classical Be Stars" consolidates the state-of-the-art understanding of Be stars, providing insights into their characteristics, mechanisms influencing their circumstellar environments, and their broader astrophysical implications.

Definition and Properties of Classical Be Stars

Classical Be stars are rapidly rotating B-type main sequence stars that exhibit variable emission lines due to the presence of a circumstellar disk composed primarily of gas in Keplerian motion. These stars rotate at about 75% of the critical velocity, a threshold beyond which material can be ejected to form a disk. This high rotation is coupled with either non-radial pulsations or small-scale magnetic activity, suggesting multiple mechanisms could be responsible for disk formation in different stars. The paper emphasizes that once the material has been lifted into orbit, memory of its ejection process is lost, and the disk's dynamics are governed by viscosity—a key characteristic of viscous decretion disk models.

Observational and Theoretical Insights

1. Pulsation and Rotation: Be stars are the most rapid non-degenerate stellar rotators known, and their pulsational properties offer insight into their internal structures. Investigations reveal a uniform rotational average across spectral types, challenging previous notions that suggested a variation in rotational speed along with the spectral type.
2. Disk Structure and Dynamics: The paper details that the disks are governed by viscous decretion, characterized by a radial density fall-off described by power laws. The dynamics are complex, including properties such as disk truncation in binary systems, driven by tidal interactions, and the presence of global oscillations causing long-term $V/R$ variability.
3. Magnetic Fields: Strong, organized magnetic fields are not definitively detected in Be stars, imposing constraints on magnetic disk formation models. Small-scale magnetic fields, however, could still play a role in localized mass ejections.
4. Interferometry and High-Resolution Observations: Techniques such as interferometry underscore the disk’s geometry, providing constraints on scales and orientations not previously attainable. These observations align with theoretical predictions and enhance our understanding of the link between rapid rotation and disk physical properties.
5. Influence of Metallicity and Rotation on Evolution: Extrinsic factors such as metallicity affect the rotation rates of Be stars. Observations in the Magellanic Clouds and theoretical models suggest higher rotation rates at lower metallicities, positing intriguing implications for the evolutionary pathways of these stars.
6. Binary Interactions and High-Energy Phenomena: The presence of companions in Be systems introduces interactions that can modify disk properties through truncation and potentially contribute to observed high-energy phenomena, such as X-ray emissions in Be/X-ray binaries.

Implications and Future Directions

The detailed insights into the rotational velocities, pulsational properties, and magnetic fields in Be stars open pathways for future spectroscopic analyses to refine these observations. Furthermore, understanding the evolutionary implications of rotational and metallicity effects could elucidate the role of Be stars in broader stellar populations, including their contribution to cosmic events like gamma-ray bursts. The paper anticipates that advancements in observation techniques, such as high-resolution spectroscopy and multi-wavelength interferometry, will further unravel the complexities of these fascinating stellar objects, serving as laboratories to probe the physics of rapid stellar rotation, disk dynamics, and angular momentum transport in massive stars. The synthesis presented establishes a baseline for ongoing and future research endeavors to refine our understanding of these enigmatic stars.